I'm looking for something that you might call a mirror distro targeted for ARM (or generic, if at all possible).

This disto is made of a target installation, and a host development installation. On the host (your x86) side you'll have a sandbox where you can emerge the whole system (crosscompilation, not emulation) with your compiler/mcpu/CFLAGs of choice. Here all the development headers will be placed.
From this host sandbox you can then create binary packages that you can easily deploy on the target distro, in such a way, that the packages are stripped of all the non-essentials.

I think that Gentoo is best suited for this approach. What steps need to be taken to make this?

It is, but this would be only the "host" part. As I understand it, this will only build & install the packages locally.

However, I need the packages to be post-processed. Man pages, locales, examples, docs, everything non-essential must be removed. From my experience with debian arm upwards of 75% of the total used space is non-essential, and space is at a premium.

Second, these processed and binary packages must be "emerged" on the target machine with ease. Hence my remark about a "mirrored" distro.

For example, the smallest "stock" debian (lenny/arm) weighs in at >400MB. 75% of this is taken up in documentation, examples, locale, /usr/share entries, (unused) cache, and inefficient dependencies.

I believe its possible to have a perfectly normal gentoo base system smaller < 100MB. Possibly even smaller than 32M without needing to resort to busybox or ulibc.

well, Gentoo is already quite smaller but as a source based distro you'll have development stuff.
Maybe you could try CRUX/ARM too or if you are looking for other ARMHF solutions, like Gentoo already offers, you can ask on: powerdeveloper.org ._________________LinuxPPC user!
my blog: http://linuxpowerpc.blogspot.com/